(1) Field of the Invention
The invention relates to the general field of chemical vapor deposition with particular reference to the deposition of fluorinated silicon oxide films.
(2) Description of the Prior Art
As dimensions in integrated circuits grow ever smaller and as circuit speeds grow faster, the demands made on the materials from which they are formed grow ever more stringent. An example of this is the inter-metal dielectric layer (IMD) that is used to separate different levels of inter-circuit wiring. To minimize the incidence of mutually induced signals between two such levels it is necessary to make the IMD as thick as possible and its dielectric constant as low as possible.
In practice, considerations such as planarity and via plug integrity limit the maximum IMD thickness so attempts have been made in recent years to find alternative dielectrics to silicon dioxide which, till now, has been the material of choice for IMDS. Since silicon dioxide already has a low dielectric constant (about 4), the choice of materials is very limited.
A promising candidate to replace silicon dioxide as an IMD has been fluorinated silicon glass, known as FSG or SiOF. FSG films are known to have a dielectric constant of 3.2-3.6, depending on the fluorine concentration. The problem addressed by the present invention is to determine the best conditions under which to form FSG films. While many different ways that differ in detail have been described, they all depend on precursor gases that decompose under the influence of heat into fluorine, silicon and oxygen. The oxygen component is usually made more reactive through ionization/dissociation in a high density plasma (HDP) and/or by the inclusion of ozone in the mix. Depending on the exact conditions used, films of varying quality will be obtained. One such variable is step coverage with or without trapped voids internal to the film. Some examples include:
a) Fukuda et al. in "Highly reliable SiOF film formation using high density plasma containing hydrogen" in Proceedings of the 1997 DUMIC Conference February 10-11 pp. 41-48 describe using inorganic sources such as silane and silicon tetrafluoride together with a plasma generator and hydrogen. They were able to achieve films having dielectric constants between 3.3 and 3.6. PA1 b) Laxman and Hochberg in "Remote microwave plasma enhanced CVD of fluorine doped silicon dioxide from FASI-4 and FTES" in Proceedings of the 1997 DUMIC Conference February 10-11 pp. 57-63 used fluorotriethoxysilane (FTES) in combination with an organosilicon compound and plasma activated oxygen to form SiOF films with about 8 atomic % fluorine and having dielectric constants around 3.6. PA1 c) Instead of an organo silicon compound, Laxman and Hochberg also used TEOS (tetra-exthyloxysilane) Si(OC.sub.2 H.sub.5).sub.4 !. PA1 d) Homma in "Characteristics of SiOF films formed using TEOS and FTES at room temperature by CVD" in J. Electrochem. Soc. vol. 143 1996 pp. 707-711 describes a process for undoped silicon oxide deposition at temperatures as low as 25.degree. C. and atmospheric pressure using a hydrolysis reaction of a TEOS-FTES mixture with water vapor. Polymer-like film structures were found due to silicon oligomer formation during the hydrolysis reaction. PA1 e) Instead of room temperature polymerization of TEOS and FTES with water vapor, Homma, in his patent (U.S. Pat. No. 5,288,518 February 1994) describes a CVD method for forming fluorine containing silicon oxide films at temperatures as low as 200.degree. C., using decomposition of alkoxysilane and fluoroalkoxysilane vapor in the presence of ozone, or under ultraviolet radiation, or in a gas plasma. The resulting films were found to have a composition of SiO.sub.1.85 F.sub.0.15. Boron or phosphorus-doped films were supposedly also formed by introduction of boron or phosphorus precursors in the gas mixture. PA1 f) Dobuzinsky et al. present a method of forming a fluorosilicate glass by reacting an O.sub.2 /Si precursor gas, a F.sub.2 precursor gas, and hydrogen. The aim is to avoid the incorporation of C or N in the film. F concentration is controlled through the hydrogen which reacts with, and thus removes from the reaction site, excess F. PA1 g) Kubo et al. in "An SiO.sub.2 film deposition technology using tetraethylorthosilicate and ozone for interlayer metal dielectrics" in J. Electrochem. Soc. vol. 143 1996 pp. 1769-1773, describe the deposition of SiO.sub.2 films (no fluorine) that exhibit good void free step coverage. Fujino et al. in "Silicon dioxide deposition by atmospheric pressure and low temperature CVD using TEOS and ozone" in J. Electrochem. Soc. vol 137 1990 pp. 2883-2887 report on the characteristics of films deposited in this way. Again, no fluorine was involved. PA1 h) Qian et al. (U.S. Pat. No. 5,571,576 November 1996) form SiOF films using plasma CVD and no ozone, while Jang et al. (U.S. Pat. No. 5,536,681 July 1996) shows a method of oxide formation wherein a plasma treatment is followed by ozone/TEOS deposition.
Note that all low temperature (around 200.degree. C. and below) deposition methods for silicon dioxide have at least two well known, but nevertheless serious, disadvantages which make them unusable in today's reality. The first problem is structural recombination of material on heating above the original temperature of formation, which is a typical treatment in the manufacture of real devices. It leads to undesirable phenomena such as substantial shrinkage and stress in films, fluorine outdiffusion (followed by changes in dielectric constant), etc. The second problem is the phenomenon of enhanced moisture absorbtion by low temperature deposited silicon dioxide films which causes problems of instability of film properties.
As pointed out above, there is a wide range of conditions under which fluorinated silicon oxide films may be deposited. It has, however, been found that in many cases the resulting films are not entirely satisfactory. In TABLE I below we summarise some of the pros and cons of some of these methods:
TABLE I ______________________________________ Formation method Advantages Disadvantages ______________________________________ Inorganic sources Excellent coverage Possibility of plasma Hi density plasma and gap-fill damage problems Microwave plasma Reduced plasma damage Non-conformal step due to remote plasma coverage and gap-fill Possible voids; low deposition rates Room temperature No plasma damage Poor oxide quality; hydrolysis of TEOS problems Changes in film and FTES properties with heating; Enhanced moisture absorption phenomena; Poor step coverage and gap-fill for submicron devices Low deposition rate Low temperature No plasma damage Poor oxide quality; decomposition of problems (without Change of properties aloxisilanes and plasma or ultra- with film heating; fluoroalkoxysilanes violet radiation) Enhanced moisture in the presence of absorption phenomena; ozone, under Possibility of plasma ultraviolet damage problems with radiation or gas plasma or ultraviolet acti- plasma; vation; Non-conformal Phosphorus/boron step coverage and gap doping of oxide is fill, possible voids in also possible submicron devices under plasma or ultraviolet; Increase of dielectric constant with dopants Ozone based SiO.sub.2 Excellent step cover Surface sensitivity CVD and gap-fill; of chemical reactions No plasma damage prob. ______________________________________
Thus, none of the existing methods for preparing SiOF films are totally satisfactory even though many experimenters have tried a wide range of deposition conditions. As we will describe below, excellent films can be obtained but only over a relatively narrow range of deposition conditions.